Cytoskeletal proteins and their regulation by Ca2+ are being studied to understand the role of these proteins in cell motility. The interactions of actin with smooth muscle and nonmuscle myosins are regulated by phosphorylation of one of the myosin subunits by specific Ca2+ calmodulin dependent kinases. Turbidity changes, ultracentrifugation, and electron microscopy were used to determine the effect of this phosphorylation on myosin filament formation. While phosphorylation does effect the degree of filament formation, the type of anion used to adjust the ionic strength is equally important, and it is unlikely that this is the principle mechanism by which phosphorylation regulates the participation of these myosins in force development. The effects of this phosphorylation on myosin subunit interactions is also being examined. Brain spectrin, fodrin, appears to link the cytoskeleton to the cell membrane. The affinity of fodrin for calmodulin binding are being investigated. The binding of actin to fodrin and its effect on calmodulin binding are being investigated. The binding of fodrin to actin is inhibited by myosin, and fodrin modulates the actomyosin ATPase.